In a typical conventional automatic transmission for an automobile, the rotation of an engine is input via a torque converter, varied in speed by a shift mechanism having a plurality of planetary gears, and output to a drive shaft or a propeller shaft (the axle side).
The shift mechanism of this type of automatic transmission executes a shift by transmitting the rotation of an input shaft to a specific gear or carrier of the planetary gear in accordance with a shift position and transmitting the rotation of the specific gear or carrier to an output shaft appropriately. The shift mechanism also comprises a plurality of frictional elements such as clutches and brakes to converge the rotation of the specific gear or carrier appropriately during the shift, and performs a predetermined shift by switching a torque transmission path in accordance with engagement and disengagement combinations of the frictional elements. Hydraulic clutches and brakes, the engagement state of which is controlled through the supply and discharge of oil pressure, are typically employed as the frictional elements.
If a vehicle travels in the vicinity of a boundary region of a vehicle traveling condition when performing a predetermined shift in a conventional automatic transmission, the selected gear position may vary such that the shift is repeated. For example, when performing a 3-4 shift from a third speed to a fourth speed, the 3-4 shift from the third speed to the fourth speed and a 4-3 shift from the fourth speed to the third speed are repeated such that the gear position varies continuously from three to four to three to four and so on.
When shifts are performed continuously in this manner, the same frictional elements are repeatedly engaged and disengaged over a long time period, and therefore the thermal load applied to the frictional elements increases (the temperature increases). As a result, the burns may occur on the frictional elements, leading to eventual burnout. It should be noted that in this specification, “thermal load” is used to mean “temperature” or “heat generation”.
In response to this problem, JP3402220B, published by the Japan Patent Office, discloses a technique using a timer. More specifically, a timer is counted down during a continuous shift, and when the timer value reaches a predetermined value, subsequent shifts are prohibited, assuming that the thermal load state (temperature) of the frictional element has reached a burnout temperature. When the continuous shift ends before the timer value reaches the predetermined value, the timer is counted up on a fixed gradient, assuming that heat radiation is underway.
Hence, when the continuous shift resumes immediately after the end of the continuous shift, countdown of the timer value begins from a smaller value than an initial value, and therefore control is executed taking into consideration the amount of accumulated heat in the frictional element.